1. Field of the Invention
The present invention relates to a semiconductor device, and more specifically to an MOS semiconductor device using a polysilicon layer as an interconnection layer.
2. Description of the Related Art
Generally, in semiconductor devices, or in MOS semiconductor devices in particular, a polysilicon layer formed by doping impurities is often used as an electrode or an interconnection layer. Further, aluminum is usually used as the interconnection material. Accordingly, a polysilicon layer is electrically connected to an aluminum layer in such a way that the polysilicon interconnection layer and the aluminum interconnection layer are laminated, disposing an interlayer insulating film therebetween, and then both the layers are connected to each other via a hole formed in the insulating film.
In more detail, in a conventional semiconductor device, an insulating film is formed to cover a polysilicon layer, a contact hole is formed in the insulating film, an aluminum layer is formed on the insulating film, and the polysilicon layer is connected to the aluminum layer via the contact hole.
With the advance of semiconductor manufacturing technology, recently there exists a tendency that the difference in level in an interlayer insulating film on which an interconnection layer is formed can be reduced and additionally the film thickness of the polysilicon layer can be reduced owing to the progress of micromanufacturing technology.
Further, although conventionally the contact holes have been formed by wet etching technique, recently dry etching is often used to form the contact hole.
As described above, in order to form a contact hole for interconnection between the two layers, it is necessary to form a desired contact hole in the insulating film formed on the surface of the polysilicon layer by dry etching. In this process, it is indispensable to etch only the insulating film without etching the polysilicon layer under the insulating film, by appropriately determining etching gas and etching conditions. In general, however, since the dry etching conditions are determined so that the insulating film can be removed perfectly for providing a secure connection, by utilizing a difference in etching rate between the insulating film and the polysilicon layer, it is unavoidable that the polysilicon layer is etched even slightly.
In the conventional device, as described above and as shown in FIG. 1, where a polysilicon layer 6 is reduced in thickness, there exists a problem in that the contact hole is formed passing through the polysilicon layer 6 into a field isolation oxide film 3 under the polysilicon layer 6, without being stopped at the polysilicon layer 6, as depicted on the right side in FIG. 1.
When an aluminum layer is formed on the upper layer, the electric contact between the aluminum interconnection layer and the polysilicon layer cannot be achieved normally, with the result that the contact resistance increases or, in the worst case, both the layers are not connected electrically.